1. Your Birth Control

You might think taking birth control should cause more regularity in your period, but women who take extended-cycle birth control pills like Seasonique, Seasonale, or Quartette won't experience menstruation on a typical 28-day cycle. That's because these contraceptives delay the amount of time between periods. "When you keep taking the active pills for more than the usual 21 days, the lining of the uterus stays stable," says George Patounakis, M.D., Ph.D., FACOG, Progyny’s fertility specialist in Florida. "Once you take the inactive pill, hormone levels drop and trigger menses. It's not a period the way you would think of in a woman who's not taking birth control because it's induced by medication and not by normal processes." Which means you might not experience typical bleeding.

Other types of hormonal birth control like intrauterine devices (IUDs) and the Depo-Provera shot can also cause late and irregular periods. Dr. Patounakis cautions that no contraceptive is 100 percent effective at preventing pregnancy. So if you don't get your period three or four days after starting the inactive portion of the pill pack, for example, you should take a pregnancy test.

2. Polycystic Ovary Syndrome (PCOS)

During a typical menstrual cycle, each ovary develops roughly five follicles, and those follicles compete to become the dominant one that will release a mature egg at ovulation. Women with PCOS often have additional follicles, which makes this process take longer than usual. No released egg means no period.

Other PCOS symptoms include weight gain and increased levels of the testosterone-like hormone androgen, which can cause thick hair growth on the face and breasts. But even without these symptoms, one can't rule out PCOS. "There are women who are not overweight and don't have extra hair who have irregular cycles and an ultrasound will show they have excessive follicles," says Anuja Vyas, M.D., FACOG, with Houston Methodist Obstetrics and Gynecology Associates.

3. Stress

How stress affects the menstrual cycle is highly subjective because what's considered stressful depends on the woman, says Dr. McDonald. Moving across the country or dealing with a challenging work situation could throw off one woman's cycle by a week or even cause it to arrive early, but have no effect on another woman.

Stress can interfere with the hypothalamus, causing a trickle-down effect. "Emotional distress can affect the region of the brain that controls the pituitary gland, which regulates the hormones that stimulate our ovaries," explains Dr. Vyas.

4. Fluctuating Weight

Excessive weight loss is more likely to cause a late period than weight gain, though increasing body weight, when related to other conditions like PCOS, can have a similar effect. "A body mass index (BMI) under 20 creates a starvation-ish mode in the brain," says Dr. McDonald. "That's why some really lean female athletes don't have periods—being underweight creates an environment that's anti-pregnancy."

"Severe weight loss and anorexia can shut down the hypothalamus's production of follicle stimulating hormone (FSH) and luteinizing hormone (LH) that regulate the ovaries," adds Dr. Vyas.

PCOS patients may be especially sensitive to the numbers on the scale. "As little as 10 percent weight loss can get them back into their cycle after experiencing irregularity," explains Dr. McDonald. "And a similar percentage of weight gain can cause a late or missed period."

5. Perimenopause

"The average age women in the U.S. experience menopause is 51, but many women start having delayed menstrual cycles in their late 40s," says Dr. Vyas. So instead of the standard 28 days between periods, menses may arrive 36 or 48 days apart. "If you're under 45 and your period stops completely, it's possible you're going through early menopause or experiencing premature ovarian failure," she adds.

6. A Pituitary Tumor

Though it's rare and unlikely, sometimes a prolactinoma, a type of pituitary tumor that secretes excess amounts of prolactin, the hormone that signals breast milk production, is to blame for a late period. Dr. Vyas says women who are suffering from headaches, blurry vision, and discharge from the breasts even though they're not breastfeeding, in addition to a menstrual cycle that's off, may want to get checked by their doctor for this type of tumor.

7. Diabetes and Thyroid Disease

Jay M. Berman, M.D., FACOG, chief of gynecological services at Detroit Medical Center's Harper Hutzel Hospital and professor of obstetrics and gynecology at Wayne State University, says other issues such as diabetes and thyroid disease can be associated with late or absent menstrual cycles. "Many women will, for various reasons, occasionally not ovulate and this can cause an early or delayed menses," he says. "Depending on her history, it may require further testing to determine the cause."

Lastly, it's important to note that vaginal bleeding after a late period may not be the monthly visitor you were expecting. "Anybody who experiences heavy bleeding and pain after a missed period and/or a positive pregnancy test should go to the doctor," says Dr. McDonald. "All bleeding is not a period, especially in a setting where is something is off."